The ecological significance of arbuscular mycorrhizal fungal effects on clonal reproduction in plants

The population ecology of clonal plants depends on the number and distribut ion of ramets formed during growth. Variation in clonal reproduction has pr eviously been explained by variation in effects of abiotic resource heterog eneity and by plant genotypic variation. Different co-occurring species of the mutualistic arbuscular mycorrhizal fungi (AMF) have been shown to diffe rentially alter growth traits of Prunella vulgaris, which we hypothesize wo uld lead to changes in clonal reproduction. Two experiments were carried ou t to test whether different co-occurring mycorrhizal fungi significantly in fluence clonal reproduction of P. vulgaris, whether this effect also occurs when P. vulgaris is growing in an artificial plant community, and how the effects compare with plant genotype effects on clonal growth of P. vulgaris . In the first experiment, the number of ramets of P. vulgaris, growing in a plant community of simulated calcareous grassland, was significantly affect ed by inoculation with different mycorrhizal fungi. The number of ramets pr oduced by P. vulgaris differed by a factor of up to 1.8 with different myco rrhizal fungi. The fungal effects on the number of new ramets were independ ent of their effects on the biomass of P. vulgaris. In a second experiment, 17 different genotypes of P. vulgaris were inoculat ed with different mycorrhizal fungi. There were significant main effects of genotypes and mycorrhizal fungi on clonal reproduction of P. vulgaris. The effect of different mycorrhizal fungi contributed more than the effect of plant genotype to variation in size and ramet production. However, mean sto lon length and spacer length, which determine the spatial arrangement of ra mets, were only significantly affected by plant genotype. There were no myc orrhizal fungal X plant genotype interactions on clonal growth of P. vulgar is indicating that there is no obvious evidence that selection pressures wo uld favor further coevolution between P. vulgaris and mycorrhizal fungal sp ecies. In natural communities plants can be colonized by several different AMF at the same time. The effect of the mixed AMF treatment on the growth and clon al reproduction of P. vulgaris could not be predicted from the responses of the plants to the single AMF To what extent, however, the patterns of colo nization by different AMF differ among plants in a natural community is unk nown. Since the effects of AMF on growth and clonal reproduction occur on a popul ation of P. vulgaris in a microcosm plant community and because the effects are also as great as those caused by plant genotypic variation, we conclud e that the effects are strong enough to potentially affect population size and variation of clonal plants in communities.